Forty-three percent say they believe Trump was definitely born in the United States. Another 20 percent say they think he probably was, the poll said.
Seven percent say Trump was probably or definitely born in another country, while 30 percent say they don't know enough to say.
Trump -- who makes a prospective campaign trip Wednesday to New Hampshire, home of the first U.S. presidential primary -- has questioned U.S. President Barack Obama's provenance, joining conspiracy theorists who argue Obama is not a natural-born citizen and is therefore not eligible to be president.
Some so-called birthers allege Obama was born in Kenya, not Hawaii, or that his birth certificate is a forgery. Others allege he's a citizen of Indonesia or that because he had dual British and U.S. citizenship at birth, he is not a natural-born citizen.
Chester Arthur, a Republican U.S. president from 1881 to 1885, was a British subject as well as a U.S. citizen at birth.
Trump's New Hampshire trip is expected to include eight stops, starting with a news conference at Portsmouth International Airport at Pease, a civil-military airport outside Portsmouth.
The stops are to include a fundraiser for the New Hampshire GOP, the New Hampshire Union Leader said.
The Gallup/USA Today poll Tuesday found 38 percent of Americans say Obama definitely was born in the United States and 18 percent say he probably was.
Fifteen percent say they believe he probably was born in another country and 9 percent say he definitely was born elsewhere.
A former Hawaii health department director told CNN Tuesday she had "no doubt" Obama was born in Hawaii.
Chiyome Fukino, a Republican, said she examined Obama's birth certificate many times and found it "absolutely authentic."
"He was absolutely born here in the state of Hawaii," she said, adding the certificate makes no mention of Obama's religion.
Trump speculated on Fox News Channel March 31 Obama may not want his birth certificate released because it may have said he was a Muslim.
"There is something on that birth certificate -- maybe religion," Trump said. "Maybe it says he's a Muslim. I don't know."
The Gallup/USA Today poll of 1,013 adults, taken Wednesday to Saturday, has a margin of error of 4 percentage points.
The Washington Post reported Wednesday 54 percent of Trump's $1.3 million in political contributions through the years went to Democrats.
A Post analysis of state and federal disclosure records indicated recipients include Senate Majority Leader Harry Reid, D-Nev.; Hillary Clinton when she was a New York Democratic U.S. senator; Sen. Chuck Schumer, D-N.Y.; Sen. John Kerry, D-Mass.; the late Sen. Ted Kennedy, D-Mass.; and U.S. Rep. Charles Rangel, D-N.Y., who was sanctioned by the House Dec. 2 for violating ethics rules.
Trump gave $50,000 to former Obama Chief of Staff Rahm Emanuel for his recent run to become Chicago's mayor, the Post reported.
Trump's office had no comment. Trump told Fox News April 14 he supported Democrats because he had few viable Republican options in New York.
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